Question: In history class, the girl to boy ratio is $7$ to $4$. If there are a total of $55$ students, how many girls are there?
Answer: A ratio of $7$ girls to $4$ boys means that a set of $11$ students will have $7$ girls and $4$ boys. A class of $55$ students has $5$ sets of $11$ students. Because we know that there are $7$ girls in each set of $11$ students, the class must have $5$ groups of $7$ girls each. There is a total of $35$ girls in history class.